The invention relates to an underwater station for pumping a well flow, comprising a separator to separate the well flow into liquid (oil/water) and gas, a pump assembly comprising a pump with a motor, and a compressor assembly comprising a compressor with a motor, and fluid carrying conduits between separator and pump, and compressor, respectively.
Offshore oil and gas production to day commonly is carried out as follows:
Production wells are drilled from a platform down into the hydrocarbon reservoir. The platform is positioned above the height of waves on a substructure which stands on the sea floor or is afloat. Well head valves which shut off the reservoir pressure, are placed on the platform, commonly straight above the production wells.
Oil which is present under high pressure in the hydrocarbon reservoir, contains much dissolved gas. The capability of the oil to hold such dissolved gas will decrease with decreasing pressure and increasing temperature. When oil flows up through the production well from the reservoir and past the well head valve on the platform, resulting in decreasing pressure, gas is, thus, released from the oil. On top of the well head valve a mixture of oil and gas (in fact a mixture of liquid (oil/water and gas)) will, thus, emerge.
This mixture of liquid and gas is transported to a processing plant which is generally provided on the platform. The function of the processing plant is mainly to separate oil and gas and to make the oil suitable for transport and the gas suitable for transport or to be returned into the reservoir.
Since the process requires energy, and hydrocarbons are inflammable, a number of auxiliary functions and emergency systems are required about the processing plant. Furthermore, operation of processing, auxiliary, and emergency systems requires operators who, in turn, need accommodation and a number of other functions. Plants, thus, tend to be large and expensive both as regards investment and operation. On great sea depths the cost problem is even greater when the platform with the plant is to be provided on an expensive substructure which is anchored to the sea floor or afloat.
Large developing projects aiming at reduced cost are underway at present. Among others, technology was developed to permit the well head valves to be placed on the sea floor--so called underwater production plants. This is of great economic importance, because the number of platforms required to drain a hydrocarbon reservoir may be reduced. An underwater production plant is placed above an area of the hydrocarbon reservoir which cannot be reached by the aid of production wells from the platform.
Production wells of an underwater production plant are drilled from floating or jackup drilling vessels. Oil and gas from the hydrocarbon reservoir flow upwards and past the well head valves on the sea floor, and then flow in the shape of a two-phase flow (oil and gas in a mixture) in a pipeline which connects the underwater production plant with the platform. Such two-phase flows will entail slugs of liquid causing hard impacts of liquid, uncontrolled flow conditions, and considerable pressure drop in the pipeline. Consequently, the distance between the underwater production plant and the platform must not be large. At present, a practical limit is considered to be approximately 15 km.
Technical solutions which might increase this distance will have a great economic potential. The extreme consequence might be that the platform becomes redundant, with the well head valves placed on the sea floor close to the hydrocarbon reservoir, and the processing, auxiliary, and emergency systems provided on land.
Extensive developing projects are underway at present to solve the problem of conveying oil/gas mixtures across large distances. There are, thus, approaches to provide the mixture of oil and gas with pressure by placing two-phase pumps on the sea floor to compensate for pressure drop. Other approaches involve separation of oil and gas on the sea floor to permit oil and gas to be pumped in separate pipelines to a processing plant. Oil and gas are then provided with the necessary energy for efficient transport to the terminal. Liquid and gas are conveyed in separate pipelines, but the liquid and gas pipelines may, if desired, converge into a multi-phase conduit, if this is deemed optimal.
Production from a number of wells may be collected to be conveyed in a common flow. A problem in this connection is occurrence of different well flow pressure. This problem may be solved by conducting the well flows, via separate stations where the well flow pressure is adapted to a common value, after which the well flows are combined in a manifold station for further transport.
The invention was developed especially in connection with the demand for pumping a well flow from offshore petroleum fields to the shore. Transport of an unprocessed well flow across great distances to land-based processing plants offers great potential profit. By placing as much as possible of the heavy and bulky processing plant on land, optimal design is much more at option since there are no longer limitations as to weight and space like the limitations found on fixed and, especially, floating platforms.
To be able to transport a well flow across great distances to the shore or to existing processing platforms where there is surplus capacity, underwater pumping stations will be required. There are a number of advantages in placing such stations on the sea floor. Compressors and pumps will be located in the middle of a coolant (sea water) of substantially constant temperature. The hazard of explosions is eliminated and the plant will not be affected by wind and waves and it will not be covered with ice. Great saving may be achieved in connection with platform costs, quarter costs and transport of staff and equipment to and from land.
There are, however, certain disadvantages and unsolved problems in connection with underwater pumping stations. Simple daily inspection and maintenance will, thus, be impossible. Systems and components for adjustment and monitoring remote underwater stations involve untried technology. Necessary electrical power must be transmitted across great distances and connection with equipment of the underwater station must be achieved in a satisfactory manner.
All equipment and all components must be high quality and show a high degree of reliability. Maintenance must be arranged according to predetermined systems, permitting replacement of equipment. As mentioned, the present invention was developed especially in connection with the demand for a pumping station which can pump a well flow from the field and to a terminal ashore or on a nearby platform. In this connection a special object of the invention is to permit simple mounting and dismantling of a pump unit on the sea floor. Mounting and dismantling should be possible by the aid of unattended diving vessels and/or hoisting devices which are surface controlled. Service/maintenance which should occur when complete units are replaced, should be possible at desired intervals of at least 1-2 years. Control and adjustment of operations should be kept at a minimum and, preferably, it should be possible to make do without monitoring the station during operation.